Information about the structural organization of the 70 proteins and the 3 RNAs of the rat liver ribosome will be sought by (1) dissociating it into its two subunits, (2) unfolding each subunit into a ribonuclease-sensitive, inactive form by treatment with EDTA, formamide, or urea, and (3) subfractionating the particles that result from the cleavage of the RNAs by the endogenous ribonucleases for identification of the proteins and fragments of RNA that they contain. In other experiments we shall fragment the RNA of the active subunits by exogenous ribonucleases, e.g., RNase A or T. Formaldehyde-fixed aliquots of all the subparticles so obtained will be characterized as to size and composition by analytical ultracentrifugation. Both the proteins and the RNAs of the subparticles will be recovered after treatment with 6M guanidine-HC1. The proteins will be examined and sized by 2-dimensional polyacrylamide gel electrophoresis, and when feasible, isolated for further characterization by column chromatography. To identify them we shall use radioactively-labeled reagents to chemically-modify specific groups, such as SH, at various stages in the fractionation. We shall investigate the EDTA-mediated redistribution of proteins with the aim of elucidating the interactions among them and with the RNA. To characterize the RNA fragments we shall examine their sizes by electrophoresis in polyacrylamide gels and their compositions by "fingerprinting" after iodination with 125I. Since cleavage of the RNA strands is required for the subfractionation we shall study the specificities of the endogenous ribonucleases of rat liver ribosomes.